Dimers of aromatic compounds are useful as raw materials in various industrial applications. For example, alkali metal salts of biphenyl-3,4,3',4'-tetracarboxylic acid are extremely useful as a raw material for heat-resistant polyimide resins.
Conventionally known processes for the production of biphenyl compounds through dehalogenation-dimerization reaction of aromatic halogen compounds include, for example, a method in which an aromatic halogen compound is subjected to dehalogenation-dimerization reaction in the presence of a palladium catalyst, water, and methanol (JP-B-59-14015 (the term "JP-B" as used herein means an "examined Japanese patent publication")), and modifications of the above method such as a method in which a polyhydric alcohol or formaldehyde is used in place of methanol (JP-A-62-26238 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), a method in which formic acid or a formic acid salt is used in place of methanol (Synthesis Communications, p. 538 (1978), JP-A-61-137838 and JP-A-61-167642), and a method in which carbon monoxide is used in place of methanol (JP-A-61-293932).
Further, the present inventors previously found that extremely high catalytic activity can be imparted to palladium catalysts for use in dehalogenation-dimerization reactions by treating the catalysts with a hydrohalogenic acid (U.S. Pat. No. 4,900,843).
These methods, however, are defective in that the conversion of the aromatic halogen compound is still insufficient or that even if conversion is satisfactory in the beginning stage of the reactions, the lives of the catalysts are too short. Hence, there has been a desire for development of a catalyst which enables intended biphenyl compounds to be industrially produced in high yields over a prolonged period of time.